This invention relates to a solid electrochromic display cell. Such a cell uses an electrochromic material which produces coloration by the application of an electric field.
Well known electrochromic materials are tungsten oxide and biologens. The use of viologens as an electrochromic material is described in U.S. Pat. No. 3,806,229 to Schoot et al. In recent years, these materials have been used for information display devices. A typical structure of a display device comprises: a pair of glass transparent panels; a transparent display electrode on one of the panels; an electrochromic material coating such as tungsten oxide deposited on the display electrode; a counter electrode on the other panel; and, a liquid electrolyte layer such as sulfuric acid filled between the electrochromic coating and the counter electrode. Upon applying an electrical potential across these electrodes, the coating changes from a transparent state to a color state, and the coloration is retained even after removal of the electrical potential. Reversing the polarity will cause the electrochromic layer to bleach to a colorless state.
Electrochromic display cells have advantages which are inherent of a passive display; that is, observation without eye fatigue is achieved and wider visual angles are permitted while viewing the cell. Furthermore, electrochromic cells are able to retain coloration after removal of the electrical potential. The cells of the prior art, however, have the disadvantage of reduced operational life when a liquid electrolyte is used. A typical reason for reduced life is that after numerous applications of electrical potential, the electrochromic coating or layer becomes eroded (i.e., peeled and dissolved). Erosion is particularly promoted at those portions of the electrochromic layer located on the edge of the display electrode where the electric field concentrates.
As the electrochromic layer thereby peels, the display electrode will be exposed causing it to dissolve into the liquid electrolyte. As a result, the connection of the electrode to outside driving circuit will be severed and the display cell will lose its information display function.
Several methods have been practiced in the prior art to prevent the electrochromic coating from eroding into the liquid electrolyte. For example, an erosion proof insulating layer has been used to cover the electrochromic coating portion. The insulating layer protects the electrochromic layer since any erosion that occurs will first affect the insulating layer. However, after the insulating layer has eroded, erosion of the electrochromic layer will then occur. A further example is the use of a liquid electrolyte saturated with a solid electrochromic material; after the solid material dissolves in the liquid, however, it will subsequently settle and return to its original solid state. While in this solid state the above mentioned problem of erosion will then occur. Consequently, this method does not resolve the problem, it merely delays, for a short time, its occurrence.
On the other hand, the use of an electrochromic display cell having a solid electrolyte is free from the disadvantage of liquid electrolyte cells mentioned above. The first type is made of an insulating material such as SiO, CaF.sub.2 or MgF.sub.2 ; these insulating materials contain impregnation water as shown in U.S. Pat. No. 3,521,941 (Deb et al). The second type is made of an ion conductive layer such as .beta.-Al.sub.2 O.sub.3, Agl or NaZrSiPO.sub.4 in which Na ions or Ag ions are used as charge carriers as shown in the following U.S. Pat. Nos. 3,712,710 (Castellion et al), 3,971,624 (Bruesch et al), and 4,106,862 (Bayard). The first type, however, produces instability of the impregnating water by creating ionization; consequently, the water will be contaminated by moisture in the air. Thus, the operation of the cell will be adversely effected. The second type, moreover, requires an undesirably high electric driving potential, and will respond slowly to change the coloration and bleaching of the electrochromic material due to the slow mobility of Na and Ag ions.
The instant invention overcomes the above disadvantages by the use of a lithium (Li) ionic conductive electrolyte. In particular, the instant invention produces an electrochromic display cell having the characteristics of high stability, relatively low driving voltage and quick response. Although Li ionic conductivity has been described in high energy or power density battery systems, no suggestion has been offered for use in other structures or devices. See "Material Research Bulletin", Vol. 13, pp. 23-32, 1978 to Boukamp et al. In fact, this reference specifically points out the limited applications of Li ionic conductivity due to its alleged relatively low stability. The instant invention is directed to the use of Li.sub.3 N, Li.sub.2+x C.sub.1-x B.sub.x O.sub.3, Li.sub.4+x +Si.sub.1-x P.sub.x O.sub.4 and Li.sub.5+x Ag.sub.1-x Si.sub.x O.sub.4 (where 0&lt;x&lt;1) in an electrochromic display cell and its improved characteristics and performance, including high stability, relatively low driving voltage and quick response.